Vagn Gylding Holmboe (1909-1996) began formal music training at the Royal Danish Academy of Music in Copenhagen at the age of 17. Three years later he moved to Berlin where he studied with Ernst Toch. Eventually he moved back to Copenhagen to teach at the Conservatoire in Copenhagen.
He composed over 200 works including over 20 concertos (his Trombone Concerto Op. 52 is particularly cool!), 13 symphonies, 21 string quartets and numerous other pieces including a pair of sonatas for guitar. So he was very versatile.
He may not be as "undiscovered" as some other composers as he has his own LastFM channel, where you can hear a number of his pieces. His Piano Concerto No. 1, Op 17 composed in 1939 has elements of Prokofiev and early Lutosławski. While there are elements of angular lines, for the most part Vagn Holmboe is firmly in the tonal world. He definitely is a mid-20th century composer based on the pieces I heard. Even his Symphony No. 13, written in 1994, has more in common with mid century tonal composers (like Britten or Shostakovich). However, that should in no way diminish his music. In many respects his music (on first listen) is as exciting and intriguing as many modern composers - undeniably worth a listen!
If you're a trombonist (like me) you will want to check out the Danish National Symphony Orchestra "Romantic Trombone Concertos" released by Dacapo and available at Naxos or Amazon. There are several trombone concertos by several different composers and a nice collection of some thrilling music.